Vibration damping devices are used to absorb vibrational energy in machines of nearly every description and use. Unless a damping device is used, vibrational energy is directly transmitted to support and surrounding structures of the machine, which can be disadvantageous. To be useful, a damping device is required to maintain high resiliency and other performance characteristics under a wide range of temperature and other environmental conditions, for a relatively long period of time.
Examples of vibration damping devices include formed shapes, mountings, harnesses, rings, bushings, and belts used to isolate sources of vibration. Natural rubber is often used in vibrational damping applications because it provides high resiliency and tear resistance. However, natural rubber does not last long under conditions of extreme heat (i.e., >120° C.) or under ozone concentrations as found in many automotive applications.
Ethylene-based elastomers such as ethylene-propylene (alpha-olefin)-diene (EPDM) elastomers are generally polymers of very high molecular weight (as measured by their Mooney viscosities) and are often suitable for use in high resilience applications. However, high molecular weight EPDM elastomers inherently possess very high viscosities, e.g., Mooney viscosity greater than 200 ML(1+4@125° C.). This inherent characteristic of EPDM results in difficulties related to the processability of these polymers. Such polymers are not processable when having Mooney viscosities above about 100 ML(1+4@125° C.). To remedy the concerns associated with the high viscosity of high molecular weight EPDM, extender oil is often added to the polymers to the reactor effluent containing the polymers to reduce the apparent viscosity. The presence of extender oil may, however, render oil extended EPDM difficult to mix and compound in some applications.
The required level of extender oil depends on the molecular weight of the elastomer, but is usually sufficient to reduce the apparent viscosity of the oil extended EPDM to a Mooney viscosity of about 100 ML(1+4@125° C.) or below. Commercially-available very high molecular weight EPDMs, which would be useful in high resilience applications such as vibrational damping, typically contain from about 50 to about 125 phr extender oil.
While extender oils improve processability during manufacturing, elastomers extended with oil are limited in compounding latitude. The amount or the type of extender oil may be undesirable in the compound recipe, and thus may restrict further compounding options for the material. Additionally, extender oils consume valuable plant throughput capacity. Thus, polymer architectures that minimize the presence of extender oil are beneficial.
Examples of EPDM polymers and processes for making them include U.S. Pat. No. 3,884,993, which is directed to a method for improving the processability and ozone resistance of EPDM elastomers, the method comprising the steps of blending solutions of separately formed low and high molecular weight polymers, where the high MW fraction has a Modified Mooney viscosity, MML (1+8 @ 150° C.) of greater than 100, and the low MW fraction has a number average molecular weight Mn below 25,000, which corresponds to a Mooney viscosity of less than about 20 ML(1+4@125° C.). The ratio of the low to the high MW fraction is 0.7 to 1.3. The compositions are produced by a process using parallel reactors.
U.S. Pat. No. 4,078,131 is directed to EPDM compositions consisting of a low molecular weight fraction having an intrinsic viscosity from 0.8 to 1.5 dl/g, and a high molecular weight fraction having an intrinsic viscosity of 3.5 to 7 dl/g, which are prepared by 2 reactors connected in series. The fractions each have a broad molecular weight distribution. The low MW fraction represents from 30 to 85% of the total polymer composition. These compositions are reported to be useful to provide a balance between green strength and tack in tire building. The intrinsic viscosity of the low molecular weight fraction would correspond to a Mooney viscosity of less than about 25 ML(1+4@125° C.) for a polymer of broad molecular weight distribution.
U.S. Pat. No. 5,677,382 is directed to ethylene-alpha-olefin-non-conjugated diene copolymer compositions reported to have improved processability. The polymers comprise a low molecular weight component and a high molecular weight component. The low molecular weight component has a Mooney viscosity of 10-150 ML(1+4@100° C.), and the high molecular weight component has a Mooney viscosity of 100-500 ML(1+4@100° C.). The ratio of the low molecular weight component to the high molecular weight component is 51/49 to 95/5. The low molecular weight component has an alpha-olefin content of 30-60 wt %, and the iodine number ratio of the low molecular weight component to the high molecular weight component is at least 4/1.
Solvent is generally removed from metallocene-based processes utilizing flash evaporation of the solvent under vacuum, wherein reduced pressure is applied to the reaction product. However, at least a portion of the oil present in the reaction product may become entrained in the solvent being removed under reduced pressure, and may be removed along with the solvent. Metallocene-based processes thus do not allow for the introduction of extender oil into the final reaction product until after the solvent has been removed by flash evaporation.
Metallocene-based processes may be limited to a polymer product having an overall Mooney viscosity of less than about 90 ML (1+4@120° C.) in the absence of extender oil, due to the handling characteristics of such polymers including the difficulties of further processing polymers having a Mooney viscosity above about 90 ML (1+4@120° C.). However, polymer compositions having a Mooney viscosity of less than or equal to about 90 ML (1+4@120° C.) in the absence of extender oil have inferior properties, in particular flex fatigue of a cured compound.
WO 00/26296 is directed to ethylene-alpha-olefin elastomeric composition made by a series reactor operation in which the high molecular weight component has a Mooney viscosity less than or equal to 120, and is present in an amount no greater than 50 wt. %.
WO 2003 066725A2 is directed to bimodal EPDM polymer compositions, comprising a major polymer fraction having a Mooney viscosity above 120 ML(1+4@125° C.), and a minor polymer fraction having a Mooney viscosity of 120 ML(1+4@125° C.) or less, where the composition has a tan delta of 0.5 or less (125° C./10.4 rad/s). These compositions are essentially free of extender oil and preferably have a Mooney viscosity below 100 ML(1+4@125° C.) to ensure ease of processability. These compositions are especially useful for resilient applications such as vibration damping devices. The compositions are prepared using a series reactor process wherein the high molecular weight component is produced in the first reactor, and the low molecular weight component is produced in the second reactor, both using metallocene catalysts. Both components have relatively narrow molecular weight distributions with a polydispersity index (Mw/Mn) of less than 4, preferably less than 3. In addition, both components have a relatively high average branching index factor of greater than 0.7, preferably greater than 0.8, on a scale in which a branching index of 1 represents a linear polymer. However, upon curing, the compositions appear to be deficient in flex fatigue resistance, which is a measure of the ability of the material to perform in dynamic applications.
Accordingly, there exists a need in the art for copolymer compositions, in particular, ethylene-alpha-olefin elastomer compositions, which comprise improved elasticity, processability and flex resistance. Embodiments of the multimodal polymer composition produced according to the instant disclosure include economically produced synthetic materials that can have one or more of the following advantages over previously known materials: improved balance of processability, resilience, durability, and reduced quantities of extender oil.